1. Field of the Invention
The present invention relates to a fluid tank and, more particularly, it relates to a fluid tank provided with a bubble removing device for removing (air) bubbles in fluid.
2. Description of the Related Art
Many cylinders and other components of construction machines are normally driven by means of hydraulic fluid. Therefore, construction machines are mostly provided with hydraulic circuits for driving cylinders. Such hydraulic circuits comprise hydraulic tanks, pumps for feeding hydraulic fluid from working tacks under pressure, oil coolers for cooling hydraulic fluid and control valves and are often provided additionally with bubble removing devices for removing bubbles produced in the hydraulic circuit. If bubbles exist in hydraulic fluid, a phenomenon called cavitation can take place to damage the pump. Therefore, bubbles are removed from the hydraulic fluid that is being returned from the cylinder or some other component to the hydraulic tank by the bubble removing device. Then, hydraulic fluid that is free from bubbles is fed back by the pump, using pressure.
Various types of bubble removing device are known.
For example, firstly, there is a type called cyclone type (see, inter alia, Japanese Patent Application Laid-Open Publication No. 2-52013) that is adapted to generate a swirling current (vortex) in hydraulic fluid, drive bubbles having a small specific gravity toward the center and separate them by way of a dedicated flow path.
Secondly, there is a type that is also referred to as cyclone type but adapted not to separate bubbles from hydraulic fluid by way of a dedicated flow path. Instead, it is adapted to cause both bubbles and hydraulic fluid to flow into the hydraulic fluid stored in the hydraulic tank and drive off only bubbles having a small specific gravity upward along the wall surface of the bubble removing device (see, inter alia, Japanese Utility Model Application Laid-Open Publication No. 61-124701).
Thirdly, there is a type having a spiral flow path of hydraulic fluid that contains bubbles and adapted to expel bubbles as they are driven toward the center while hydraulic fluid is made to pass through the flow path (see inter alia, Japanese Patent Application Laid-Open Publication No. 56-83602).
However, all the above listed types are accompanied by problems specific to each of them.
Since a bubble removing device of the first type is arranged outside the hydraulic tank, it requires a large space dedicated to the hydraulic system because a bubble removing device has to be installed there in addition to a hydraulic tank and other facilities. Additionally, the bubble removing device has to be installed somewhere in the middle of the piping system and hence the installing operation will be a cumbersome and time consuming one.
In the case of using a bubble removing device of the second type, both bubbles and hydraulic fluid from which bubbles are expelled are forced to flow into the hydraulic fluid already stored in the tank. Therefore, if the hydraulic tank is rocked to a large extent as a result of a change to the posture of the construction machine, for instance, some or all of the bubbles that have been separated from hydraulic fluid can be mixed with the later to reduce the bubble removing effect of the device.
Finally, in the case of using a bubble removing device of the third type, since the flow path of hydraulic fluid that contain bubbles is realized to show a spiral form, the helix has to be made to show a large diameter so as to generate a sufficiently large centrifugal force and reliably expel bubbles. Then, the entire bubble removing device needs to have a large dimension in a radial direction of the helix so that consequently it requires a large space.